Combination for filtrate control and gas migration

ABSTRACT

The present invention relates to the use of a combination of block polymers and particular compositions in a fluid injected under pressure into an oil-bearing rock, where: the fluid comprises solid particles and/or is brought into contact with solid particles within the oil-bearing rock following the injection thereof, the combination comprises (i) a polymer comprising: —a first block that is adsorbed on at least a portion of the particles; and —a second block, having a composition different from that of the first, and having a weight-average molecular weight of greater than 10 000 g/mol, for example greater than 100 000 g/mol, and that is soluble in the fluid; (ii) particles suitable for providing a gas barrier effect, preferably a latex and/or silica particles.

This application is a U.S. national phase entry under 35 U.S.C. § 371 ofInternational Application No. PCT/EP2018/058364, filed Mar. 30, 2018,which claims priority to French Application No. 1752840, filed on Apr.3, 2017. The entire contents of these applications are explicitlyincorporated herein by this reference.

The present invention relates to the field of oil extraction. Morespecifically, it relates to agents providing an effect of controllingfluid loss and migration of liquids and gases in fluids injected underpressure into subterranean formations.

In the field of oil extraction, numerous stages are carried out byinjecting fluids under pressure within subterranean formations. In thepresent description, the notion of “subterranean formation” isunderstood in its broadest sense and includes both a rock containinghydrocarbons, in particular oil, and the various rock layers traversedin order to access this oil-bearing rock and to ensure the extraction ofthe hydrocarbons. Within the meaning of the present description, thenotion of “rock” is used to denote any type of constituent material of asolid subterranean formation, whether or not the material constitutingit is strictly speaking a rock. Thus, in particular, the expression“oil-bearing rock” is employed here as synonym for “oil-bearingreservoir” and denotes any subterranean formation containinghydrocarbons, in particular oil, whatever the nature of the materialcontaining these hydrocarbons (rock or sand, for example).

Mention may in particular be made, among the fluids injected underpressure into subterranean formations, of the various fluids forcompletion and workover of the wells, in particular drilling fluids,whether they are used to access the oil-bearing rock or else to drillthe reservoir itself (drill-in), or else fracturing fluids, oralternatively completion fluids, control or workover fluids or annularfluids or packer fluids.

A specific case is that of oil cement grouts, which are employed for thecementing of the annulus of oil wells according to a method well-knownper se, for example described in Le Forage [Drilling] by J. P Nguyen(Editions Technip 1993). These oil cement grouts are injected underpressure within a metal casing introduced into the drilling hole of theoil wells, then rise again, under the effect of the pressure, via the“annulus” space located between the casing and the drilling hole, andthen set and harden in this annulus, thus ensuring the stability of thewell during drilling.

Within an oil extraction well, bringing the fluid under pressure intocontact with the subterranean formation (which generally exhibits a moreor less high porosity, indeed even cracks) induces a “fluid loss”effect: the liquid present in the fluid has a tendency to penetrate intothe constituent rock of the subterranean formation, which can damage thewell, indeed even harm its integrity. When these fluids employed underpressure contain insoluble compounds (which is very often the case, inparticular for oil cement grouts or else drilling or fracturing fluidscomprising polymers), the effect of fluid loss at the same time bringsabout a concentration of the fluid, which can result in an increase inviscosity, which affects the mobility of the fluid.

In the specific case of a cement grout, the fluid loss can in additionresult in excessively rapid setting of the cement, before the space ofthe annulus is cemented, which can, inter alia, weaken the structure ofthe well and harm its leaktightness.

For further details relating to the effect of fluid loss and itscementing effects, reference may in particular be made to WellCementing, E. B. Nelson (Elsevier, 1990).

For the purpose of inhibiting the phenomenon of fluid loss, a number ofadditives have been described which make it possible to limit (indeedeven in some cases completely prevent) the escape of the liquid presentin the fluid toward the rock with which it comes into contact. Theseadditives, known as “fluid loss control agents”, generally make itpossible to obtain, in parallel, an effect of control of the migrationof gases, namely isolation of the fluid with respect to the gasespresent in the rock (gases which it is advisable to prevent frompenetrating into the fluid, in particular in the case of cement grouts,these gases having a tendency to weaken the cement during setting).

Within the meaning of the present invention and unless explicitlyindicated otherwise, “fluid loss” is understood to mean both aphenomenon of liquid migration and a phenomenon of gas migration.

Various fluid loss control agents of the abovementioned type have beenprovided, which include in particular cellulose derivatives (for examplehydroxyethylcellulose) or alternatively AMPS-based copolymers, such asthose described, for example, in U.S. Pat. No. 4,632,186 or 4,515,635.These additives are not always fully suitable for providing, inpractice, effective limitation of fluid loss. In particular, and this isespecially the case in the field of oil cement grouts, the presence ofother additives can inhibit the effect of the agents employed forproviding control of fluid loss. In particular, in the presence of somedispersing agents or set retarders, the abovementioned fluid losscontrol agents generally experience a deterioration in their properties.

An aim of the present invention is to provide novel fluid loss controlagents for fluids injected under pressure into subterranean formationswhich are well-suited in practice.

To this end, the present invention proposes to use a specificcombination including (i) specific copolymers, which are capable ofproviding a fluid loss control effect when they are employed withparticles, with which they combine, (it being possible for theseparticles to be particles present within the subterranean formation;and/or cement particles in the case of a fluid employed in cementing;and/or particles injected within subterranean formations with thecopolymers) and (ii) a specific composition inducing a gas-barriereffect, typically a latex or silica particles.

More specifically, according to a first aspect, a subject matter of thepresent invention is the use, as fluid loss and/or gas migration controlagent in a fluid (F) injected under pressure into a subterraneanformation,

where said fluid (F) comprises solid particles (p) and/or is broughtinto contact with solid particles (p) within the subterranean formationsubsequent to its injection,

of a combination comprising:

(i) a block polymer (P) comprising:

-   -   a first block (A) which is adsorbed on at least a portion of the        particles (p); and    -   a second block (B), with a composition distinct from that of        said first block (A), with a weight-average molecular weight of        greater than 10 000 g/mol, for example of greater than 100 000        g/mol, and which is soluble in the fluid (F),        (ii) particles capable of providing a gas-barrier effect,        preferably a latex and/or silica particles.

According to one embodiment, the particles capable of providing agas-barrier effect are latex.

The latex is typically in the form of a suspension containing from 35%to 60%, typically from 40% to 55%, for example from 40% to 50%, byweight, of dry latex.

The ratio of concentration of polymer (P) with respect to that of thedry latex is between 4% and 8% by weight.

In the combination of the polymer (P) with a latex of the inventionwhich makes it possible to employ less latex in order to provide agas-barrier effect, generally, the amount of dry latex is from 1.5% to6% by weight, with respect to the amount of solid particles (p)containing in the fluid (F), typically oil cement grout. Preferably, theamount of dry latex is greater than 2%, typically greater than 2.2%,indeed even greater than 2.5%, and less than 5.5%, or for example lessthan 4.5%, indeed even less than 4%, by weight, with respect to theamount of solid particles (p) containing in the fluid (F), typically oilcement grout.

Generally, the amount of polymer (P) is from 0.1% to 0.5% by weight,with respect to the amount of solid particles (p) containing in thefluid (F), typically oil cement grout. Preferably, the amount of polymeris greater than 0.2% and less than 0.4% by weight, with respect to theamount of solid particles (p) containing in the fluid (F); for example,the amount of polymer (P) can be from 0.2% to 0.4% or from 0.25% to 0.4%by weight, with respect to the amount of solid particles (p) containingin the fluid (F).

The latex is typically made of styrene/butadiene having astyrene/butadiene ratio by weight of between 30:70 and 70:30, preferablybetween 40:60 and 60:40, suspended in an aqueous solution.

The aqueous solution can additionally comprise a stabilizer for thelatex, for example a surfactant, such as is well known in this field. Anexample of the latex suspensions is described in EP 0 091 377.

When the polymer (P) is combined with a styrene/butadiene latex in anoil cement grout, it turns out that the combination provides bothcontrol of fluid loss and control of gas migration.

According to another embodiment, the particles capable of providing agas-barrier effect are particles of silica, typically in the amorphousform.

The silica particles are typically dispersed or in the form ofaggregates consisting of individual particles of 5 nm to 5 μm,preferably of 50 nm to 500 nm.

The silica particles can, for example, be in the form of a silica fume.

This is because, when the polymer (P) is combined with the silicaparticles in an oil cement grout, it turns out that the combinationprovides both control of fluid loss and control of gas migration.

Generally, the amount of silica is from 1.5% to 6% by weight, withrespect to the amount of solid particles (p) containing in the fluid(F), typically oil cement grout. Generally, the amount of silica isgreater than 2%, typically greater than 2.2%, indeed even greater than2.5%, and less than 5.5%, for example less than 4.5%, indeed even lessthan 4%, by weight, with respect to the amount of solid particles (p)containing in the fluid (F), typically oil cement grout.

Generally, the amount of polymer (P) is from 0.1% to 0.5% by weight,with respect to the amount of solid particles (p) containing in thefluid (F), typically oil cement grout. More advantageously, the amountof polymer (P) is greater than 0.2% and less than 0.4% by weight, withrespect to the amount of solid particles (p) containing in the fluid(F); for example, the amount of polymer (P) can be from 0.2% to 0.4% orfrom 0.25% to 0.4% by weight, with respect to the amount of solidparticles (p) containing in the fluid (F).

Within the meaning of the present invention, typically, the first block(A), also known as “short block” hereinafter, has a weight-averagemolecular weight typically of less than 30 000 g/mol, which is adsorbed,preferably irreversibly, on at least a portion of the particles (p); anda second block (B), also known as “long block” hereinafter, has acomposition distinct from that of said first block and a weight-averagemolecular weight of greater than 10 000 g/mol, for example of greaterthan 100 000 g/mol, and which is soluble in the fluid (F).

The specific polymer employed in the context of the present invention,due to the presence of the two specific blocks (A) and (B), turns out toprovide a particularly efficient effect of control of the fluid: thepresence of the block (A) provides anchoring of the polymer to theparticles and the presence of the long block (B), which is large in sizeand soluble, schematically provides an effect of local increase in theviscosity of the fluid (F) around the particles.

There is thus obtained, at the surface of the particles (p), theformation of a polymer layer based on the long blocks (B) anchored tothe particles using the blocks (A), the particles/polymers combinationthus produced forming, as it were, a “plug” of sufficient size withrespect to the porosities of the rock, which makes it possible to limit,indeed even to completely block, the phenomenon of fluid loss.

It should be noted that the use of polymers based on long blocks (B)alone would not provide control of fluid loss according to theinvention, which requires anchoring of the long blocks (B) to theparticles (p) via the short blocks (A), as is illustrated in theexamples given at the end of the present description.

In particular, in order for this anchoring to be as effective aspossible, it is preferable for the interaction between the short block(A) and the particles (p) to be as strong as possible and advantageouslyfor this interaction to be irreversible. Preferably, the short block (A)of a polymer (P) of use according to the invention comprises:

-   -   at least one chemical group forming at least one bond of ionic,        covalent or ionocovalent type between polymer and particle;        and/or    -   several chemical groups each forming at least one hydrogen        and/or Van der Waals bond between polymer and particle, the        combination of these bonds together forming an overall bond with        a force at least in the range of that of a bond of ionic,        covalent or ionocovalent type.

In addition, the strong interactions between particles and polymers makeit possible, if need be, to employ the polymer (P) in the presence ofadditives which are normally harmful to the effectiveness of the fluidloss control agents. In particular, the polymers (P) as employedaccording to the invention can be employed in the majority of theformulations of fluids intended to be injected into oil-bearing rocks,in particular oil cement grouts comprising additives of dispersant orset retarder type, as well as in drilling fluids and fracturing fluids.

According to a first alternative form of the invention, the injectedfluid (F) comprises the polymer (P) but does not comprise solidparticles (p), and it encounters said particles (p) within thesubterranean formation subsequent to its injection. The associationbetween particles and polymers then takes place in situ. Such a fluidcan, for example, be injected during a drilling operation, and the rockcuttings formed during the drilling then perform the role of theparticles (p) in situ.

According to a variant alternative form, the injected fluid (F)comprises, before the injection, at least a portion and generally all ofthe particles (p) combined with the polymer (P), it being understoodthat it can optionally encounter other particles (p) within thesubterranean formation.

Two forms can in particular be envisaged in this context:

-   -   form 1: the polymer (P) and the particles (p) are mixed during        the formulation of the fluid (F), on the site of operation or        upstream, typically by adding the particles (p), in the dry        state or optionally in the dispersed state, to a composition        comprising the polymer (P) in solution. According to this        alternative form, the fluid (F) can, for example, be an oil        cement grout, which is prepared by adding cement powder as        particles (p) to an aqueous composition comprising the        polymer (P) in solution.    -   form 2: the fluid (F) is manufactured, advantageously on the        site of operation, from a composition (premix) prepared upstream        (hereinafter denoted by the term “blend”) comprising the        polymer (P) and at least a portion of the particles (p),        generally within a dispersing liquid. In order to form the fluid        (F), this blend is mixed with the other constituents of the        fluid (F).        In the context of these forms 1 and 2, the polymer (P)        incidentally exhibits the not insignificant advantage of        improving the dispersibility and the suspending of the particles        (p). In some embodiments, the polymers (P) combined with the        particles (p) can be employed mainly as dispersing and        stabilizing agent for the dispersion of the particles (p), at        the same time providing an effect of agent for control of fluid        loss.

According to another aspect, the subject matter of the present inventionis certain specific block polymers which can be used according to theinvention, which were developed by the inventors specifically for thisapplication.

Another subject matter of the invention is the fluids for injectionunder pressure within an oil-bearing rock or a drilling well comprisingthese specific block polymers, and also the blends for the preparationof these fluids comprising these polymers in combination with particles(p).

Various specific advantages and embodiments of the invention will now bedescribed in more detail.

The Fluid (F) and the Long Block (B)

The term “fluid” is understood to mean, within the meaning of thedescription, any homogeneous or non-homogeneous medium comprising aliquid or viscous vector which optionally transports a liquid or gelleddispersed phase and/or solid particles, said medium being overallpumpable by means of the devices for injection under pressure used inthe application under consideration.

The term “liquid or viscous vector” of the fluid (F) is understood tomean the fluid itself, or else the solvent, in the case where the fluidcomprises dissolved compounds, and/or the continuous phase, in the casewhere the fluid comprises dispersed elements (droplets of liquid orgelled dispersed phase, solid particles, and the like).

The nature of the fluid (F) and of the long block (B) of the polymers(P) used according to the present invention can vary to a fairly largeextent, subject to the compatibility of the liquid or viscous vector ofthe fluid (F) and of the long block (B). In particular, use is made of along block (B) of hydrophilic nature when the liquid or viscous vectorpresent in the fluid (F) is of hydrophilic nature; conversely, when theliquid or viscous vector of the fluid (F) is hydrophobic, use is made ofa long block (B) of hydrophobic nature.

The long block (B) of the polymers of use according to the invention isspecifically soluble in the fluid (F). This is understood to mean thatthe long block (B), taken in isolation, can be dissolved in the liquidor viscous vector of the fluid (F). Preferably, the long block (B) issoluble at 25° C. and at 1% by weight in the liquid or viscous vector ofthe fluid (F). The notion of “solubility at 25° C.” implies only that itis possible to obtain a more or less viscous, indeed even gelled,solution which, at 25° C., does not result in precipitation. This notiondoes not exclude the possibility of the dissolution of the block (B)involving prior heating to more than 25° C. in order to obtain thissolution. In other words, the notion of “solubility at 25° C.” impliesthe possibility of forming a solution which does not precipitate at 25°C. and not the possibility of forming, at 25° C., a solution which doesnot precipitate.

Furthermore, it is preferable for the long block (B) to develop thefewest possible interactions, indeed even no interactions at all, withthe particles (p). Furthermore, it is preferable for the long block (B)of the polymers (P) of use according to the invention to develop fewerinteractions with the particles than the short block (A).

In any case, the block (A) and the block (B) have distinct compositions.This is understood to mean that:

-   -   the blocks (A) and (B) comprise distinct monomer units;    -   or    -   at least some of the monomers present on the block (A) are not        present on the block (B); and/or at least some of the monomers        present on the block (B) are not present on the block (A);    -   or    -   the block (A) and the block (B) comprise the same monomer units        but in distinct proportions.

According to a highly suitable embodiment, the fluid (F) is an aqueousfluid. The term “aqueous” is understood here to mean that the fluidcomprises water as liquid or viscous vector, either as sole constituentof the liquid or viscous vector or in combination with otherwater-soluble solvents.

In the case of the presence of solvents other than water in the liquidor viscous vector of the fluid (F), the water advantageously remains thepredominant solvent within the liquid or viscous vector, advantageouslypresent in a proportion of at least 50% by weight, indeed even of atleast 75% by weight, with respect to the total weight of the solvents inthe liquid or viscous vector.

When the fluid (F) is an aqueous fluid, the block (B) is advantageouslya block of hydrophilic nature. The term “block of hydrophilic nature” isunderstood here to mean a polymer block which, in the isolated state, issoluble in pure water in a proportion of 1% by weight at 25° C. (itbeing possible for the dissolution to optionally involve heating),forming a more or less viscous, indeed even gelled, solution but withoutformation of precipitate at 25° C.

Advantageously, the block (B) of hydrophilic nature employed when thefluid (F) is an aqueous fluid is at least predominantly composed ofmonomer units selected from the group consisting of the monomer units U1to U5 defined below, and the mixtures of these monomer units:

-   -   monomer units U1: monomer units comprising an acrylamide, in        particular dimethylacrylamide (DMA), or else (meth)acrylamide,        morpholine N-oxide acrylamide or diacetone acrylamide functional        group; the block (B) advantageously comprises monomer units of        this type.    -   monomer units U2: monomer units comprising a sulfonic acid or        sulfonate functional group, including in particular the        3-sulfopropyl (meth)acrylate, 2-propene-1-sulfonic acid, sodium        1-allyloxy-2 hydroxypropylsulfonate (COPS1), in particular        2-acrylamido-2-methylpropanesulfonic acid (AMPS), (meth)allyl        sulfonate, sodium vinylsulfonate, sodium styrenesulfonate,        (3-sulfopropyl)dimethyl(3-methacrylamidopropyl)ammonium,        N-(2-methacryloyloxyethyl)-N,N-dimethyl-N-(3-sulfopropyl)ammonium        betaine or N-(2-1-(3-sulfopropyl)-2-vinylpyridinium betaine        units.    -   monomer units U3: neutral monomer units including, inter alia:        -   esters of α,β-ethylenically unsaturated mono- or            dicarboxylic acids with C₂-C₃₀ alkanediols or polyethylene            glycols, for example 2-hydroxyethyl acrylate, 2-hydroxyethyl            methacrylate, 2-hydroxyethyl ethacrylate, 2-hydroxypropyl            acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl            acrylate, 3-hydroxypropyl methacrylate, 3-hydroxybutyl            acrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutyl            acrylate, 4-hydroxybutyl methacrylate, 6-hydroxyhexyl            acrylate, 6-hydroxyhexyl methacrylate,            3-hydroxy-2-ethylhexyl acrylate, 3-hydroxy-2-ethylhexyl            methacrylate, N-(hydroxymethyl)acrylamide,            N-(2-hydroxypropyl)methacrylamide, N-hydroxyethylacrylamide,            N-[tris(hydroxymethyl)methacrylamide, 4-acryloylmorpholine,            2-(N-morpholino)ethyl methacrylate, polyethylene glycol            meth(acrylate), diethylene glycol (meth)acrylate, ethylene            glycol methyl ether (meth)acrylate, 2-hydroxyethyl acrylate,            hydroxypropyl acrylate, poly(propylene glycol) acrylate,            2-chloroethyl acrylate,        -   tetrahydrofurfuryl acrylate, vinylacetamide,            vinylpyrrolidone, N-vinylpiperidone, N-vinylcaprolactam,            N-vinyl-5-methyl-2-pyrrolidone,            N-vinyl-5-ethyl-2-pyrrolidone,            N-vinyl-6-methyl-2-piperidone, N-vinyl-6-ethyl-2-piperidone,            N-vinyl-7-methyl-2-caprolactam or            N-vinyl-7-ethyl-2-caprolactam.    -   monomer units U4: monomer units carrying ammonium groups, in        particular esters of α,β-ethylenically unsaturated mono- or        dicarboxylic acids with aminoalcohols, such as        N,N-dimethylaminomethyl (meth)acrylate, N,N-dimethylaminoethyl        (meth)acrylate, N,N-diethylaminoethyl acrylate,        N,N-dimethylaminopropyl (meth)acrylate, N,N-diethylaminopropyl        (meth)acrylate and N,N-dimethylaminocyclohexyl (meth)acrylate;        -   amides of α,β-ethylenically unsaturated mono- or            dicarboxylic acids with diamines having at least one primary            or secondary amine group, such as            N-[2-(dimethylamino)ethyl]acrylamide,            N-[2-(dimethylamino)ethyl]methacrylamide,            N-[3-(dimethylamino)propyl]acrylamide,            N-[3-(dimethylamino)propyl]methacrylamide,            N-[4-(dimethylamino)butyl]acrylamide,            N-[4-(dimethylamino)butyl]methacrylamide,            N-[2-(diethylamino)ethyl]acrylamide,            N-[4-(dimethylamino)cyclohexyl]acrylamide or            N-[4-(dimethylamino)cyclohexyl]methacrylamide;            N,N-diallylamines and N,N-diallyl-N-alkylamines, including            in particular            (3-sulfopropyl)dimethyl(3-methacrylamidopropyl)ammonium,            N-(2-methacryloyloxyethyl)-N,N-dimethyl-N-(3-sulfopropyl)ammonium            betaine, N-(2-1-(3-sulfopropyl)-2-vinylpyridinium betaine            and N-(2-1-(3-sulfopropyl)-4-vinylpyridinium betaine.    -   monomer units U5: acrylate monomer units carrying a COOH or COO—        group, including in particular acrylic acid, methacrylic acid,        ethacrylic acid, α-chloroacrylic acid, crotonic acid, maleic        acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic        acid, aconitic acid, fumaric acid or monoethylenically        unsaturated C₄-C₁₀ and preferably C₄ to C₆ dicarboxylic acid        monoesters, such as monomethyl maleate. According to one        possible embodiment, the block (B) of hydrophilic nature        employed when the fluid (F) is an aqueous fluid is composed        essentially, indeed even exclusively, of monomer units of the        abovementioned type,        where, in the abovementioned units, the acid groups can be, in        all or part, in the free acid form and/or in the salt form, for        example potassium, sodium or ammonium salt form (neutralized        form).

As employed in the present description, the expression “polymer orpolymer block at least predominantly composed of monomer units ‘x’”denotes a homopolymer or copolymer (block) resulting from thepolymerization of a mixture of monomers, including monomers thishomopolymer or copolymer (block) comprising less than 25 mol %,preferably less than 15 mol % and more advantageously still less than 10mol % of monomer units other than the units ‘x’.

The expression “polymer or polymer block essentially composed of monomerunits ‘x’” for its part denotes, within the meaning of the presentdescription, a homopolymer or copolymer (block) at least predominantlycomposed of monomer units ‘x’ of the abovementioned type, morespecifically comprising less than 5 mol %, preferably less than 2 mol %and more advantageously still less than 1 mol % of monomer units otherthan the units ‘x’.

According to a specific embodiment compatible with the precedingembodiments, the block (B) of hydrophilic nature employed when the fluid(F) is an aqueous fluid can comprise hydrophobic monomers in smallproportions, typically in a proportion of at least 0.05%, inparticularly at least 0.1%, indeed even at least 0.5%, if appropriate;this content of hydrophobic monomers preferably remaining below 10%, forexample below 5%, in particular below 3%, indeed even 2%, thesepercentages being expressed by weight with respect to the total weightof monomer units in the block (B). When hydrophobic monomers of thistype are present, they can typically (but nonlimitingly) be chosen fromalkyl acrylates (such as methyl acrylate), styrene, alkyl methacrylatesand/or vinyl acetate.

The long block (B) present in the polymers employed according to thepresent invention furthermore has a weight sufficiently great to providethe desired effect of controlling fluid loss. To this end, the block (B)typically has a weight-average molecular weight of greater than 100 000g/mol, preferably of greater than 150 000 g/mol, for example of greaterthan 200 000 g/mol, in particular of greater than 250 000 g/mol, thisbeing the case in particular when the block (B) is of one of theabovementioned types. In practice, this weight-average molecular weightgenerally remains below 3 000 000 g/mol (and typically between 150 000and 2 000 000 g/mol) but higher weights can be envisaged in theabsolute, except in the specific case of a fluid (F) used in the contextof a cementing operation, where it is preferable for the weight-averagemolecular weight of the long block (B) to remain below 1 000 000 g/moland advantageously below 800 000 g/mol.

In the context of the present invention, it has furthermore beendemonstrated that, surprisingly, the desired effect of controlling fluidloss is obtained for blocks (B) having a lower weight-average molecularweight than 100 000 g/mol. Thus, according to a specific embodiment, theblock (B) has a weight-average molecular weight of between 10 000 and100 000 g/mol, preferably of at least 20 000 g/mol, for example of atleast 25 000 g/mol, it being possible for this weight-average molecularweight to be typically less than 90 000, for example less than 75 000,indeed even less than 50 000.

An estimation of the weight-average molecular weight of the long block(B) can be measured by size exclusion chromatography and measurement ofweight using external calibration with polyethylene oxide standards(relative SEC), which results in a slightly increased value of theweight-average molecular weight denoted in the present description byMw(relative SEC).

This Mw(relative SEC) is typically measured under the followingconditions:

-   -   Mobile phase: Mixture of 80% by weight of deionized water,        additivated with 0.1M NaNO₃, and 20% by weight of acetonitrile    -   Flow rate: 1 ml/min    -   Columns: Shodex OHpak SB 806 MHQ (3×30 cm columns)    -   Detection: Refractive index (Agilent concentration detector)    -   Concentration of the samples: approximately 0.5% by weight of        solids in the mobile phase    -   Injection: 100 μl    -   Internal reference: ethylene glycol    -   Calibration: polyethylene oxide PEO

The Mw(relative SEC) of the long block (B) of the polymers (P) of useaccording to the invention is generally greater than or equal to 125 000g/mol, preferably greater than or equal to 150 000 g/mol, thisMw(relative SEC) typically being between 200 000 and 2 500 000 g/mol, inparticular between 250 000 and 2 000 000 g/mol. According to a morespecific embodiment, it can be less than 125 000 g/mol, for examplebetween 12 500 and 100 000 g/mol.

In the case of a fluid (F) used in the context of a cementing operation,the Mw(relative SEC) of the long block (B) of the polymers (P) istypically (but nonlimitingly) between 25 000 and 900 000 g/mol, forexample between 250 000 and 900 000 g/mol.

In practice, the Mw(relative SEC) of the polymer (P) is measured, which,as a result of the low weight of the block (A), also represents a fairlygood approximation, inflated, of the weight-average molecular weight ofthe block (B). The Mw(relative SEC) of the polymer (P) is generallygreater than or equal to 15 000 g/mol, and for example greater than orequal to 150 000 g/mol, preferably greater than or equal to 200 000g/mol, for example greater than or equal to 300 000 g/mol, in particulargreater than or equal to 400 000 g/mol, this Mw(relative SEC) of thepolymer (P) typically being between 200 000 g/mol and 2 500 000 g/mol,in particular between 250 000 g/mol and 2 000 000 g/mol. It is moreparticularly between 25 000 and 900 000 g/mol, for example between 250000 g/mol and 800 000 g/mol, in the case of a fluid (F) used in thecontext of a cementing operation.

In the Specific Case where the Fluid (F) is Used in a CementingOperation

(Oil Cement Grout, Typically):

-   -   the block (B) is advantageously a block of hydrophilic nature,        preferably comprising units U1 of the abovementioned type, in        particular dimethylacrylamide DMA units, optionally but not        necessarily in combination with units U2, in particular        acrylamidomethylpropanesulfonic acid (AMPS) units, optionally in        all or part in the sulfonate form, for example in the form of        its sodium salt.    -   According to a specific embodiment, the block (B) is at least        predominantly (for example essentially, indeed even exclusively)        composed of a mixture of DMA and AMPS units, with a DMA/AMPS        molar ratio for example of between 60/40 and 90/10, in        particular between 75/25 and 85/15 and typically of the order of        80/20.    -   the block (B) typically has a weight-average molecular weight of        between 150 000 and 750 000 g/mol, preferably between 200 000        and 700 000 g/mol. Alternatively, the block (B) can have a        weight-average molecular weight of between 15 000 and 150 000        g/mol, preferably between 20 000 and 100 000 g/mol.    -   the block (B) typically has a Mw(relative GPC) of between 200        000 and 800 000 g/mol, preferably between 250 000 and 900 000        g/mol, for example from 300 000 to 600 000 g/mol, the        polymer (P) generally having a Mw(relative SEC) within these        ranges. The block (B) can alternatively have a Mw(relative GPC)        of between 20 000 and 200 000 g/mol, preferably between 25 000        and 180 000 g/mol, for example 30 000 and 150 000 g/mol, the        polymer (P) generally having a Mw(relative SEC) within these        ranges.        The long block (B) employed when the fluid (F) is an oil cement        grout is typically a random DMA/AMPS block with a DMA/AMPS molar        ratio between 75/25 and 85/15 (typically of the order of 80/20)        and a Mw(GPC-MALS) of between 20 000 and 7500 000, in particular        between 200 000 and 750 0000, for example between 400 000 and        600 000.        The Particles (p) and the Short Block (A)

The notion of “particle” within the meaning under which it is employedin the present description is not confined to that of individualparticles. It more generally denotes solid entities which can bedispersed within a fluid, in the form of objects (individual particles,aggregates, and the like) for which all the dimensions are less than 5mm, preferably less than 2 mm, for example less than 1 mm.

The nature of the particles (p) and of the short block (A) of thepolymers (P) used according to the present invention can vary to afairly large extent, provided that the block (A) interacts with theparticles (p) and results in an immobilization, preferably irreversible,of the polymer (P) on the surface of the particles (p).

To do this, the block (A) generally comprises monomer units carryinggroups which develop, with the particles (p), stronger interactions thanthe long block (B).

According to a highly suitable embodiment, the particles (p) areinorganic particles introduced within the fluid (F) or with which thefluid (F) comes into contact subsequent to its injection. Theseparticles (p) are then typically cement, calcium carbonate, clay,baryte, silica, sand or carbon black particles. According to thisembodiment, the block (A) is preferably at least predominantly (andpreferably essentially, indeed even exclusively) composed of monomerunits chosen from the preferred groups defined hereinafter, to beadjusted on an individual basis as a function of the nature of theparticles (p):

For Particles (p) of Calcium Carbonate or Cement:

-   -   the block (A) can in particular be at least predominantly (and        preferably essentially, indeed even exclusively) composed of:        -   monomer units U5 of the abovementioned type, advantageously            present in the block (A); and/or        -   monomer units U3 of the abovementioned type; and/or        -   monomer units U6 carrying phosphate, phosphonate or            phosphinate groups (in the free acid form and/or in the            saline form), such as, for example, monoacryloyloxyethyl            phosphate or bis(2-methacryloyloxyethyl) phosphate units,            the monomer units introduced by employing Sipomer PAM 100,            200, 400 or 5000 available from Solvay, vinylphosphonic            acid, allylphosphonic acid, isopropylphosphonic acid,            diallyl aminomethylene phosphonate and their salts.    -   The block (B) is then typically at least predominantly (and        preferably essentially, indeed even exclusively) composed of        units U1 and/or U2 of the abovementioned type.

For Particles (p) of Silica or Sand:

-   -   the block (A) can in particular be at least predominantly (and        preferably essentially, indeed even exclusively) composed of:        -   monomer units U3 of the abovementioned type; and/or        -   monomer units U4 of the abovementioned type; and/or        -   monomer units U7 which are (meth)acrylate units            functionalized by polydimethylsiloxanes, such as            trimethylsiloxy-terminated PEG 4-5 methacrylate or            3-(trimethoxysilyl)propyl methacrylate.    -   The block (B) is then typically at least predominantly (and        preferably essentially, indeed even exclusively) composed of        units U1 and/or U2 and/or U5 of the abovementioned type.

For Particles (p) of Clay:

-   -   the block (A) can in particular be at least predominantly (and        preferably essentially, indeed even exclusively) composed of:        -   monomer units U4 of the abovementioned type; and/or        -   monomer units U6 of the abovementioned type.    -   The block (B) is then typically at least predominantly (and        preferably essentially, indeed even exclusively) composed of        units U1 and/or U2 of the abovementioned type.

For Particles (p) of Carbon Black:

-   -   the block (A) can in particular be at least predominantly (and        preferably essentially, indeed even exclusively) composed of        hydrophobic units U8, including in particular:    -   esters of α,β-ethylenically unsaturated mono- or dicarboxylic        acids with C₁-C₂₀ alcohols, such as, for example, methyl        (meth)acrylate, methyl ethacrylate, ethyl (meth)acrylate, ethyl        ethacrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate,        n-butyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl        (meth)acrylate, tert-butyl ethacrylate, n-hexyl (meth)acrylate,        n-heptyl (meth)acrylate, n-octyl (meth)acrylate,        1,1,3,3-tetramethylbutyl (meth)acrylate, ethylhexyl        (meth)acrylate, n-nonyl (meth)acrylate, n-decyl (meth)acrylate,        n-undecyl (meth)acrylate, tridecyl (meth)acrylate, myristyl        (meth)acrylate, pentadecyl (meth)acrylate, palmityl        (meth)acrylate, heptadecyl (meth)acrylate, nonadecyl        (meth)acrylate, arachidyl (meth)acrylate, behenyl        (meth)acrylate, lignoceryl (meth)acrylate, cerotinyl        (meth)acrylate, melissinyl (meth)acrylate, palmitoleoyl        (meth)acrylate, oleyl (meth)acrylate, linoleyl (meth)acrylate,        linolenyl (meth)acrylate, stearyl (meth)acrylate, lauryl        (meth)acrylate, mono-, di- or tristyrylphenyl (meth)acrylates        optionally ethoxylated between the aromatic and methacrylate        groups; and/or        -   vinylaromatic monomer units, such as styrene,            2-methylstyrene, 4-methylstyrene, 2-(n-butyl)styrene,            4-(n-butyl)styrene or 4-(n-decyl)styrene;        -   fluorinated monomer units, such as perfluorinated or highly            fluorinated alkyl (meth)acrylates.    -   The block (B) is then typically at least predominantly (and        preferably essentially, indeed even exclusively) composed of        units U1 and/or U2 and/or U5 of the abovementioned type.

Whatever its chemical nature, the short block (A) present in thepolymers employed according to the present invention generally has aweight-average molecular weight between 500 and 30 000 g/mol, forexample between 1000 and 25 000 g/mol, this being the case in particularwhen the block (A) is of one of the abovementioned types.

According to a particularly advantageous embodiment, employed when theparticles (p) are particles of cement or calcium carbonate, the shortblock (A) is a poly(acrylic acid) homopolymer block with aweight-average molecular weight ranging from 1000 to 20 000 g/mol.

The weight-average molecular weight of the short block (A) can inparticular be measured by gel permeation chromatography, followed by amulti-angle light scattering analysis (GPC-MALS).

The Polymers (P)

The polymers of use according to the present invention are specificpolymers which comprise at least two blocks of very different size,including a large-sized block (B).

The polymers (P) are preferably prepared by controlled radicalpolymerization, which makes it possible to finely control the size ofthe two blocks.

The controlled radical polymerization technique is a technique wellknown per se which makes it possible, using a control agent for thepolymerization, to obtain polymers of controlled weights and inparticular block polymers, both the architecture and the size of each ofthe blocks of which can be controlled.

Controlled radical polymerization processes which are highly suitablefor the synthesis of the polymers (P) of use according to the inventionare the “RAFT” or “MADIX” processes, which typically employ a reversibleaddition-fragmentation transfer process employing control agents (alsoknown as reversible transfer agents), for example of xanthate type(compounds carrying —SC═SO— functional groups). Mention may inparticular be made, as examples of such processes, of those described inWO 96/30421, WO 98/01478, WO 99/35178, WO 98/58974, WO 00/75207, WO01/42312, WO 99/35177, WO 99/31144, FR 2 794 464 or WO 02/26836.

These “controlled radical polymerization” processes result, in awell-known way, in the formation of polymer chains which growsubstantially all at the same rate, which is reflected by asubstantially linear increase in the molecular weights with theconversion and a narrow distribution in the weights, with a number ofchains which remains typically substantially fixed throughout theduration of the reaction, which makes it possible to very easily controlthe mean molar mass of the polymer synthesized (the monomer/controlagent initial ratio defines the degree of polymerization obtained forthe chains synthesized). The chains obtained furthermore generallyexhibit a “living” nature: they exhibit, at the chain end, the reactivegroup present on the control agent. For this reason, it is possible tocontinue the polymerization on the polymer chain obtained, whileretaining the controlled nature of the polymerization, which can inparticular be used to synthesize, at the end of a first polymer block ofcontrolled size, another block with a different composition and also ofcontrolled size.

In this context, the polymers (P) of use according to the invention canbe polymers of the type which are prepared according to a processcomprising the following stages:

(E1) the block (A)—or more rarely the block (B)—of the polymers (P) issynthesized by bringing into contact, in an aqueous medium:

-   -   the ethylenically unsaturated monomers, which are identical or        different, chosen for the construction of the block        (A)—respectively of the block (B);    -   a source of free radicals which is suitable for the        polymerization of said monomers; and    -   a control agent for the radical polymerization, preferably        comprising a thiocarbonylthio —S(C═S)— group;        (E2) the block (B)—respectively the block (A)—at the end of the        block (A)—respectively at the end of the block (B)—formed in        stage (1) is synthesized by bringing into contact:    -   the ethylenically unsaturated monomers, which are identical or        different, chosen for the construction of the block        (B)—respectively of the block (A);    -   a source of free radicals which is suitable for the        polymerization of said monomers; and    -   the polymer obtained on conclusion of stage (E1), which acts as        control agent for the radical polymerization and onto which the        block (B)—respectively the block (A)—is grafted.

In each of stages (E1) and (E2), the size of the polymer block beingformed is controlled by the monomer/control agent molar ratiocorresponding to the initial amount of monomers with respect to theamount of control agent: schematically, all the chains grow startingfrom each of the control agents present and the monomers arehomogeneously distributed over all the growing chains. For this reason,the monomer/control agent molar ratio dictates the degree ofpolymerization of the block synthesized in each of the stages and thusmakes it possible to define the theoretical number-average molecularweight expected for each of the blocks.

Typically, the monomer/control agent molar ratios in stages (E1) and(E2) are chosen so that:

-   -   The theoretical number-average molecular weight of the block (A)        is between 250 and 25 000 g/mol, preferably between 500 and 15        000 g/mol, in particular between 1000 and 10 000 g/mol.    -   The theoretical number-average molecular weight of the block (B)        is between 70 000 and 5 000 000 g/mol, preferably between 80 000        and 3 000 000 g/mol, in particular between 90 000 and 2 000 000        g/mol. When the polymer is intended for a cementing operation,        this theoretical number-average molecular weight of the        block (B) is more preferably between 90 000 and 1 000 000 g/mol,        advantageously between 100 000 and 500 000 g/mol.

The block (B) can advantageously be prepared in stage (E2) by bringinginto contact:

-   -   the ethylenically unsaturated monomers, which are identical or        different, chosen for the construction of the block (B);    -   a source of free radicals which is suitable for the        polymerization of said monomers; and    -   the block (A) prepared according to the abovementioned stage        (E1), which acts as control agent for the radical        polymerization, preferably comprising a thiocarbonylthio        —S(C═S)— group and onto which the block (B) is grafted;        with a concentration of monomers within the reaction medium of        stage (E) which is sufficiently high to bring about the gelling        of the medium if the polymerization were carried out in the        absence of the control agent.

This polymerization technique makes it possible to access large-sizedblocks (B). Advantageously, the synthesis of the block (B) can becarried out under the polymerization conditions described in theapplication WO 2012/042167.

Alternatively, when the block (B) is hydrophilic, the block (B) can besynthesized by bringing into contact, within an aqueous medium (M) inwhich the block (B) formed is not soluble:

-   -   the ethylenically unsaturated monomers, which are identical or        different, chosen for the construction of the block (B) and        chosen to be soluble in the aqueous medium (M);    -   at least one source of free radicals; and    -   a reactive stabilizer which comprises:        -   a polymer chain (PC) which is soluble in the medium (M),        -   a group (G) providing the radical polymerization of            stage (E) with a living and controlled nature, such as, for            example, a group carrying a thiocarbonylthio —S(C═S)— group.

Generally, the conditions to be employed in the abovementionedpolymerization stages can be those typically employed in controlledradical polymerizations.

In particular, use may be made, in stage (E) of the process of theinvention, of any source of free radicals known per se. For example, oneof the following initiators may be concerned:

-   -   hydrogen peroxides, such as: tert-butyl hydroperoxide, cumene        hydroperoxide, t-butyl peroxyacetate, t-butyl peroxybenzoate,        t-butyl peroxyoctoate, t-butyl peroxyneodecanoate, t-butyl        peroxyisobutyrate, lauroyl peroxide, t-amyl peroxypivalate,        t-butyl peroxypivalate, dicumyl peroxide, benzoyl peroxide,        potassium persulfate or ammonium persulfate,    -   azo compounds, such as: 2,2′-azobis(isobutyronitrile),        2,2′-azobis(2-butanenitrile), 4,4′-azobis(4-pentanoic acid),        1,1′-azobis(cyclohexanecarbonitrile),        2-(t-butylazo)-2-cyanopropane,        2,2′-azobis[2-methyl-N-(1,1)-bis(hydroxymethyl)-2-hydroxyethyl]propionamide,        2,2′-azobis(2-methyl-N-hydroxyethyl]propionamide,        2,2′-azobis(N,N′-dimethyleneisobutyramidine) dichloride,        2,2′-azobis(2-amidinopropane) dichloride,        2,2′-azobis(N,N′-dimethyleneisobutyramide),        2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide),        2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide),        2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] or        2,2′-azobis(isobutyramide) dihydrate,    -   redox systems comprising combinations, such as:    -   mixtures of hydrogen peroxide, alkyl peroxide, peresters,        percarbonates and the like and any of iron salts, titanous        salts, zinc formaldehyde sulfoxylate or sodium formaldehyde        sulfoxylate, and reducing sugars,    -   alkali metal or ammonium persulfates, perborates or perchlorates        in combination with an alkali metal bisulfite, such as sodium        metabisulfite, and reducing sugars, and    -   alkali metal persulfates in combination with an arylphosphinic        acid, such as benzenephosphonic acid and the like, and reducing        sugars.

In particular in the case of polymerizations carried out in an aqueousmedium, use may be made of a radical initiator of redox type, whichexhibits the advantage of not requiring heating of the reaction medium(no thermal initiation), which makes it possible to manage even betterthe exothermicity of the reaction.

Thus, the source of free radicals which is employed can typically bechosen from the redox initiators conventionally used in radicalpolymerization, typically not requiring heating for their thermalinitiation. It is typically a mixture of at least one oxidizing agentwith at least one reducing agent.

The oxidizing agent present in the redox system is preferably awater-soluble agent. This oxidizing agent can, for example, be chosenfrom peroxides, such as: hydrogen peroxide, tert-butyl hydroperoxide,cumene hydroperoxide, t-butyl peroxyacetate, t-butyl peroxybenzoate,t-butyl peroxyoctoate, t-butyl peroxyneodecanoate, t-butylperoxyisobutyrate, lauroyl peroxide, t-amyl peroxypivalate, t-butylperoxypivalate, dicumyl peroxide, benzoyl peroxide, sodium persulfate,potassium persulfate, ammonium persulfate or also potassium bromate.

The reducing agent present in the redox system is also preferably awater-soluble agent. This reducing agent can typically be chosen fromsodium formaldehyde sulfoxylate (in particular in its dihydrate form,known under the name Rongalit, or in the form of an anhydride), ascorbicacid, erythorbic acid, sulfites, bisulfites or metasulfites (inparticular alkali metal sulfites, bisulfites or metasulfites),nitrilotrispropionamides, and tertiary amines and ethanolamines (whichare preferably water-soluble).

Possible redox systems comprise combinations, such as:

-   -   mixtures of water-soluble persulfates with water-soluble        tertiary amines,    -   mixtures of water-soluble bromates (for example, alkali metal        bromates) with water-soluble sulfites (for example, alkali metal        sulfites),    -   mixtures of hydrogen peroxide, alkyl peroxide, peresters,        percarbonates and the like and any of iron salts, titanous        salts, zinc formaldehyde sulfoxylate or sodium formaldehyde        sulfoxylate, and reducing sugars,    -   alkali metal or ammonium persulfates, perborates or perchlorates        in combination with an alkali metal bisulfite, such as sodium        metabisulfite, and reducing sugars, and    -   alkali metal persulfates in combination with an arylphosphinic        acid, such as benzenephosphonic acid and the like, and reducing        sugars.

An advantageous redox system comprises (and preferably consists of), forexample, the combination of ammonium persulfate and sodium formaldehydesulfoxylate.

Generally, and in particular in the case of the use of a redox system ofthe ammonium persulfate/sodium formaldehyde sulfoxylate type, it provesto be preferable for the reaction medium of stage (E) to be devoid ofcopper. In the case of the presence of copper, it is generally desirableto add a copper-complexing agent, such as EDTA.

The nature of the control agent employed in the stages for the synthesisof the blocks (A) and (B) can, for its part, vary to a large extent.

According to an advantageous alternative form, the control agent used isa compound carrying a thiocarbonylthio —S(C═S)— group. According to aspecific embodiment, the control agent can carry severalthiocarbonylthio groups.

It can optionally be a polymer chain carrying such a group. Thus, thecontrol agent employed in stage (E2) is a living polymer resulting fromstage (E1). Likewise, the control agent of stage (E1) can be envisagedas resulting from a preliminary stage (E0) in which the radicalpolymerization was carried out of a composition comprising:

-   -   ethylenically unsaturated monomers;    -   a control agent for the radical polymerization comprising at        least one thiocarbonylthio —S(C═S)— group; and    -   an initiator of the radical polymerization (source of free        radicals).

More generally, a control agent suitable for the synthesis of thepolymer (P) of use according to the invention advantageously correspondsto the formula (A) below:

in which:

-   -   Z represents:        -   a hydrogen atom,        -   a chlorine atom,        -   an optionally substituted alkyl or optionally substituted            aryl radical,        -   an optionally substituted heterocycle,        -   an optionally substituted alkylthio radical,        -   an optionally substituted arylthio radical,        -   an optionally substituted alkoxy radical,        -   an optionally substituted aryloxy radical,        -   an optionally substituted amino radical,        -   an optionally substituted hydrazine radical,        -   an optionally substituted alkoxycarbonyl radical,        -   an optionally substituted aryloxycarbonyl radical,        -   an optionally substituted acyloxy or carboxyl radical,        -   an optionally substituted aroyloxy radical,        -   an optionally substituted carbamoyl radical,        -   a cyano radical,        -   a dialkyl- or diarylphosphonato radical,        -   a dialkyl-phosphinato or diaryl-phosphinato radical, or        -   a polymer chain,            and    -   R₁ represents:        -   an optionally substituted alkyl, acyl, aryl, aralkyl, alkene            or alkyne group,        -   a saturated or unsaturated, aromatic, optionally substituted            carbocycle or heterocycle, or        -   a polymer chain.

The R₁ or Z groups, when they are substituted, can be substituted byoptionally substituted phenyl groups, optionally substituted aromaticgroups, saturated or unsaturated carbocycles, saturated or unsaturatedheterocycles, or groups selected from the following: alkoxycarbonyl oraryloxycarbonyl (—COOR), carboxyl (—COOH), acyloxy (—O₂CR), carbamoyl(—CONR₂), cyano (—CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl,arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino,guanidimo, hydroxyl (—OH), amino (—NR₂), halogen, perfluoroalkylC_(n)F_(2n+1), allyl, epoxy, alkoxy (—OR), S-alkyl, S-aryl, groupsexhibiting a hydrophilic or ionic nature, such as alkali metal salts ofcarboxylic acids, alkali metal salts of sulfonic acid, polyalkyleneoxide (PEO, PPO) chains, cationic substituents (quaternary ammoniumsalts), R representing an alkyl or aryl group, or a polymer chain.

According to a specific embodiment, R₁ is a substituted orunsubstituted, preferably substituted, alkyl group.

The optionally substituted alkyl, acyl, aryl, aralkyl or alkynyl groupsgenerally exhibit from 1 to 20 carbon atoms, preferably from 1 to 12 andmore preferably from 1 to 9 carbon atoms. They can be linear orbranched. They can also be substituted by oxygen atoms, in particular inthe form of esters, sulfur atoms or nitrogen atoms.

Mention may in particular be made, among the alkyl radicals, of themethyl, ethyl, propyl, butyl, pentyl, isopropyl, tert-butyl, pentyl,hexyl, octyl, decyl or dodecyl radical.

The alkyne groups are radicals generally of 2 to 10 carbon atoms; theyexhibit at least one acetylenic unsaturation, such as the acetylenylradical.

The acyl group is a radical generally exhibiting from 1 to 20 carbonatoms with a carbonyl group.

Mention may in particular be made, among the aryl radicals, of thephenyl radical, which is optionally substituted, in particular by anitro or hydroxyl functional group.

Mention may in particular be made, among the aralkyl radicals, of thebenzyl or phenethyl radical, which is optionally substituted, inparticular by a nitro or hydroxyl functional group.

When R₁ or Z is a polymer chain, this polymer chain can result from aradical or ionic polymerization or from a polycondensation.

In the context of the present invention, it is in particularadvantageous to employ, as control agents, xanthates, trithiocarbonates,dithiocarbamates or dithiocarbazates.

Use is advantageously made, as control agent, of compounds carrying axanthate —S(C═S)O— functional group, for example carrying an O-ethylxanthate functional group of formula —S(C═S)OCH₂CH₃, such as, forexample, O-ethyl S-(1-(methoxycarbonyl)ethyl) xanthate of formula(CH₃CH(CO₂CH₃))S(C═S)OEt.

Another possible control agent in stage (E) is dibenzyl trithiocarbonateof formula PhCH₂S(C═S)SCH₂Ph (where Ph=phenyl).

The polymers (P) of use according to the invention generally compriseonly the blocks (B) and (A). They are typically diblock polymers (A)-(B)but polymers comprising more blocks can be envisaged, in particularcopolymers comprising a long block (B) onto which two or more shortblocks are grafted (triblock polymers of (A)-(B)-(A) type inparticular), or else copolymers comprising a spacer block between theblock (B) and the block (A).

Some of the polymers of use according to the present invention arepolymers which, to the knowledge of the inventors, have never beendescribed previously.

These polymers comprise in particular the block polymers containing,preferably as sole blocks:

-   -   at least one first block pAA at least predominantly (and        preferably essentially, indeed even exclusively) composed of        acrylic acid units, with a weight-average molecular weight        between 500 and 30 000 g/mol, in particular between 1000 and 20        000; and    -   a second block p(DMA/AMPS) at least predominantly (and        preferably essentially, indeed even exclusively) composed of a        random mixture of DMA and AMPS units, with a DMA/AMPS molar        ratio of between 60/40 and 90/10 and with a weight-average        molecular weight of greater than 150 000 g/mol, typically        between 200 000 and 2 000 000 g/mol and in particular between        250 000 and 750 000 g/mol.

These polymers constitute a specific subject matter of the presentinvention, and also

-   -   the oil cement grouts comprising them,    -   the aqueous fluids for injection under pressure within an        oil-bearing rock, in particular drilling fluids and fracturing        fluids, comprising them in combination with inorganic particles,        and also the blends for the preparation of these fluids.        Practical Applications

The polymers of use according to the invention can be employed invirtually all of the fluids used in oil extraction and potentiallysubject to fluid loss.

According to a specific embodiment of the invention, the fluid (F) is anoil cement grout which comprises the polymer (P) as additive. In thiscase, the polymer (P), combined with the particles present in thecement, provides the effect of control of fluid loss during thecementing.

According to another embodiment, the fluid (F) is a drilling fluid or afracturing fluid which comprises the polymer (P) combined with particles(p). The particles (p) are then generally introduced jointly with thepolymer into the fluid (F) before the injection of the fluid. Thepolymer then generally provides stabilization of the dispersion of theparticles in the fluid (F) by keeping at least a portion of theparticles (p) in suspension in the fluid.

The concentrations of polymer and particles to be employed in thesevarious fluids can be adjusted individually as a function of theapplication targeted and of the rheology desired.

Various aspects and advantages of the invention will be furtherillustrated by the examples below, in which polymers were preparedaccording to the process of the invention.

EXAMPLE 1 Synthesis of poly(acrylic acid)-b-poly(N,N-dimethylacrylamide-co-AMPS) Diblock Copolymers

1.1: Synthesis of Living Poly(Acrylic Acid) Blocks Having a XanthateEnding (Short Blocks A1 to A4)

30 g of acrylic acid in an aqueous solvent (namely 70 g of distilledwater for the blocks A1-A3—a mixture of 35 g of distilled water and 28 gof ethanol for the block A4) and O-ethyl S-(1-(methoxycarbonyl)ethyl)xanthate of formula (CH₃CH(CO₂CH₃))S(C═S)OEt (in the amounts shown intable 1 below, where the value of the theoretical number-averagemolecular weight expected (M_(n), th), calculated by the ratio of theamount of monomer to the amount of xanthate, is also shown) and 312 mgof 2,2′-azobis(2-methylpropionamidine) dihydrochloride were introducedinto a 250 ml round-bottomed flask at ambient temperature. The mixturewas degassed by bubbling with nitrogen for 20 minutes.

The round-bottomed flask was subsequently placed in an oil baththermostatically controlled at 60° C. and the reaction medium was leftstirring at 60° C. for 4 hours.

On conclusion of these four hours, the conversion was determined by ¹HNMR.

An analysis by size exclusion chromatography in a mixture of water andacetonitrile (80/20) additivated with NaNO₃ (0.1N) with an 18-angle MALSdetector provides the weight-average molar mass (M_(w)) andpolydispersity index (M_(w)/M_(n)) values given in table 1 below.

TABLE 1 blocks A1-A4 Block M_(n), th Xanthate Conversion M_(w)synthesized (g/mol) (g) (¹H NMR) (g/mol) M_(w)/M_(n) A1 10 000  0.62499.5% 22 000  1.8 A2 5000 1.25 99.7% 10 000  1.7 A3 2500 2.50 99.6% 50001.7 A4 1000 6.24 >99.9% 2100 1.81.2: Synthesis of Diblock Copolymers from the Blocks A1 to A4 PolymersP1 to P17

The blocks A1 to A4 prepared as shown in section 1.1 were employed intheir reaction medium obtained, without purification, with a weight ofpolymer w_(A) given in table 2 below. The chosen block, in its reactionmixture without purification, was introduced into a 250 mlround-bottomed flask at ambient temperature and thenN,N-dimethylacrylamide DMA, a 50% by weight aqueous AMPS solution (25%by molar ratio to the amount of N,N-dimethylacrylamide) and distilledwater, with a final solids content of approximately 20% by weight, andammonium persulfate as a 5.0% by weight aqueous solution were added (inamounts given in table 2 below).

The mixture was degassed by bubbling with nitrogen for 20 minutes.Sodium formaldehyde sulfoxylate, in the form of a 1.0% by weight aqueoussolution, was added to the medium, the same weight of this solutionbeing introduced as that of the ammonium persulfate solution (see table2).

The polymerization reaction was allowed to take place without stirringat ambient temperature (20° C.) for 24 hours.

On conclusion of the 24 hours of reaction, the conversion was measuredby ¹H NMR (results in table 3).

An analysis by size exclusion chromatography in a mixture of water andacetonitrile (80/20 v/v) additivated with NaNO₃ (0.1N) with a refractiveindex detector provides the number-average molar mass (M_(n)) andpolydispersity index (M_(w)/M_(n)) values which are listed in table 3.

TABLE 2 polymers P1 to P17: amounts of reactants employed during thesynthesis Polymer Short block w_(DMA) w_(AMPS) w_(water) w_(persulf)w_(sfs) synthesized w_(A) (g) (g) (g) (g) (g) (g) P1 A1 0.835 15.3 17.775.6 6.0 6.0 P2 A1 1.65 15.2 17.6 75.7 6.0 6.0 P3 A1 1.65 6.16 7.12 29.13.0 3.0 P4 A1 2.68 5.06 5.85 25.4 3.0 3.0 P5 A1 3.99 5.60 6.47 27.9 3.03.0 P6 A1 5.51 5.19 6.00 27.3 3.0 3.0 P7 A2 0.426 15.2 17.6 75.6 6.0 6.0P8 A2 0.847 15.2 17.6 75.6 6.1 6.3 P9 A2 0.840 6.28 7.27 29.6 3.0 3.0P10 A2 1.59 5.93 6.86 29.6 3.0 3.0 P11 A2 2.13 5.98 6.91 29.0 3.0 3.0P12 A2 3.05 5.67 6.55 28.8 3.0 3.0 P13 A3 0.703 25.6 29.6 138.1 3.0 3.0P14 A3 1.74 25.3 29.3 137.6 3.0 3.0 P15 A4 0.177 31.7 36.6 157.5 12.012.0 P16 A4 0.287 25.6 29.6 139 3.0 3.0 P17 A4 0.709 25.3 29.3 138.7 3.03.0 P18 A4 5.33 25.3 29.3 138.7 0.5 0.5 w_(water): weight of distilledwater added, with the exclusion of the water added in the othersolutions w_(persulf): weight of the 5% by weight aqueous ammoniumpersulfate solution added w_(sfs): weight of the 1% by weight aqueoussodium formaldehyde sulfoxylate solution

TABLE 3 polymers P1 to P17 Polymer Short Conversion M_(w) synthesizedblock DMA AMPS (kg/mol) M_(w)/M_(n) P1 A1  99.4%  98.0% 620 5.2 P2 A1 99.7%  99.2% 420 3.2 P3 A1 >99.9% >99.9% P4 A1 >99.9% >99.9% P5A1 >99.9% >99.9% P6 A1 >99.9% >99.9% P7 A2  99.6%  99.8% 600 3.0 P8 A2 99.8%  99.2% 390 3.3 P9 A2 >99.9% >99.9% P10 A2 >99.9% >99.9% P11A2 >99.9% >99.9% P12 A2 >99.9% >99.9% P13 A3  99.7%  98.7% 450 2.3 P14A3 >99.9% >99.9% 210 1.9 P15 A4  99.4%  99.6% 760 2.5 P16 A4  99.8% 99.5% 410 2 P17 A4  99.7%  99.4% 180 1.9 P18 A4  >99%  >99% 47.5 1.5

EXAMPLE 2 (COMPARATIVE) Synthesis of apoly(N,N-dimethylacrylamide-co-AMPS) Monoblock Polymer

15.3 g of N,N-dimethylacrylamide, 18 g of AMPS, 75.6 g of distilledwater, 1.03 g of a 1% by weight ethanolic solution of O-ethylS-(1-(methoxycarbonyl)ethyl) xanthate of formula(CH₃CH(CO₂CH₃))S(C═S)OEt and 6.0 g of a 5% by weight aqueous solution ofammonium persulfate were introduced into a 250 ml round-bottomed flaskat ambient temperature. The mixture was degassed by bubbling withnitrogen for 20 minutes.

6.0 g of a 1% by weight aqueous solution of sodium formaldehydesulfoxylate were subsequently added. The two aqueous solutions ofammonium persulfate and sodium formaldehyde sulfoxylate had beendegassed beforehand by bubbling with nitrogen.

The polymerization reaction was then allowed to take place withoutstirring at ambient temperature (20° C.) for 24 hours.

On conclusion of the 24 hours of reaction, a conversion of 99.8% ofN,N-dimethylacrylamide and of 99.6% of AMPS was obtained, as determinedby ¹H NMR.

An analysis by size exclusion chromatography in water additivated withNaNO₃ (0.1N) with a refractive index detector provides the followingnumber-average molar mass (M_(n)) and polydispersity index (M_(w)/M_(n))values (relative to the PEO standards):

M_(w)=1 070 000 g/mol

M_(w)/M_(n)=2.8.

EXAMPLE 3 Evaluation of Diblock Polymer in Combination with a Latex inCement Grouts

The diblock polymer P13 prepared in example 1 and dried by evaporationon a plate is used in the powder form in combination with astyrene/butadiene latex to prepare oil cement grouts having thefollowing formulations, in grams:

3-1 3-2 3-3 3-4 3-5 3-6 Latex (g) 103.9 86.6 69.24 0 0 69.2 Diblock P13(g) 0 0 0 3.9 2.34 2.34 Dispersant (polymelamine sulfonate) (g) 1.951.17 1.56 0 1.56 1.17 Retarder (calcium lignosulfonate) (g) 0.39 0.390.39 0.78 0.78 0.39 Organic antifoaming agent (g) 1.38 1.38 1.38 1.381.38 1.38 Municipal water (g) 245 262 278.9 344.8 345.5 277.8 Cement (g)780 780 780 780 780 780

The latex is added to the municipal water and to the liquid additives.The diblock and the solid additives are mixed with cement beforeincorporation in the liquid.

The formulation and the conditioning and the filtration test werecarried out according to the standard of the American PetroleumInstitute (API recommended practice for testing well cements, 10B, 2ndedition, April 2013).

After mixing and dispersing all the constituents of the formulation, thegrout obtained was conditioned at 88° C. for 20 minutes in anatmospheric consistometer (model 1250 supplied by Chandler EngineeringInc.), prestabilized at this temperature, which makes it possible tosimulate the conditions experienced by the cement grout during descentin a well.

The fluid loss control performance was determined by a static filtrationat 88° C. in a double-ended cell with a capacity of 175 ml equipped with325 mesh (45 μm) metal screens (supplied by Ofite Inc., reference170-45). The gas migration control tests are carried out on a Model120-57 gas migration tester supplied by Ofite. The principle of thisappliance consists in allowing the cement grout to set under pressureduring a prolonged filtration. If no gas diffuses through the cell, thegrout is regarded as impermeable to gas migration.

The performance qualities of the polymers in the cement formulations aregiven in the table below:

Formulation No. 3-1 3-2 3-3 3-4 3-5 3-6 Fluid loss V API (ml) 85 400 38180 121 39 Gas migration control yes no no no no yes

These results show that a combination of latex and of diblock polymercan advantageously be used to control both the fluid loss and the gasmigration with a concentration of latex and a diblock which are greatlyreduced with respect to the use of each of these additives used alone.

EXAMPLE 4 Evaluation of Diblock Polymer in Combination with SilicaParticles in Cement Grouts

The diblock polymer P13 prepared in example 1 and dried by evaporationon a plate is used in the powder form in combination with a silica fume(supplied by Condensil, grade 95ND) to prepare oil cement grouts havingthe following formulations, in grams:

4-1 4-2 4-3 Silica fume (g) 39 0 35.9 Diblock P13 (g) 0 3.12 3.12Dispersant (polymelamine sulfonate) (g) 2.34 0.59 0.78 Retarder (calciumlignosulfonate) (g) 0.78 0.78 0.78 Organic antifoaming agent (g) 1.381.38 1.38 Municipal water (g) 345.1 350.2 Cement (g) 780 780 780 4-4 4-5Silica fume (g) 22.1 27.63 Diblock P13 (g) 3.9 4.88Hydroxyethylcellulose suspending agent (g) 0.325 0.325 Retarder (calciumlignosulfonate) (g) 3.25 5.85 Organic antifoaming agent (g) 1.4 1.4Municipal water (g) 365.8 366.1 Cement (g) 650 650 Silica flour (g)227.5 227.5

The silica fume is added to the municipal water and to the liquidadditives and to the municipal water. The diblock and the solidadditives are mixed with the cement before incorporation in the liquid.

The formulation and the conditioning and the filtration test werecarried out according to the standard of the American PetroleumInstitute (API recommended practice for testing well cements, 10B, 2ndedition, April 2013).

After mixing and dispersing all the constituents of the formulation, thegrout obtained was conditioned at 88° C. for 20 minutes in anatmospheric consistometer (model 1250 supplied by Chandler EngineeringInc.), prestabilized at this temperature, which makes it possible tosimulate the conditions experienced by the cement grout during descentin a well.

The fluid loss control performance was determined by a static filtrationat 88° C. (for the formulations 4-1, 4-2 and 4-3), at 120° C. (for theformulation 4-4) and at 150° C. (for the formulation 4-5) in adouble-ended cell with a capacity of 175 ml equipped with 325 mesh (45μm) metal screens (supplied by Ofite Inc., reference 170-45). The gasmigration control tests are carried out on a Model 120-57 gas migrationtester supplied by Ofite. The principle of this appliance consists inallowing the cement grout to set under pressure during a prolongedfiltration. If no gas diffuses through the cell, the grout is regardedas impermeable to gas migration.

The performance qualities of the polymers in the cement formulations aregiven in the table below:

Formulation No. 4-1 4-2 4-3 4-4 4-5 Temperature (° C.) 88 88 88 120 150Fluid loss V API 400 80 46 15 14 Gas migration control no no yes yes yes

These results show that a combination of latex and of diblock polymercan advantageously be used to control both the fluid loss and themigration in combination with a dispersion of silica fume.

The invention claimed is:
 1. A method, comprising injecting a fluid (F) under pressure into a subterranean formation, wherein said fluid (F) comprises solid particles (p) or is brought into contact with solid particles (p) within the subterranean formation subsequent to its injection, and a combination comprising: (i) a block polymer (P) comprising: a first block (A) which is adsorbed on at least a portion of the solid particles (p); and a second block (B) with a composition distinct from that of said first block (A), with a weight-average molecular weight of greater than 10,000 g/mol and less than 1,000,000 g/mol and which is soluble in fluid (F), and (ii) particles capable of providing a gas barrier effect; wherein the amount of the particles capable of providing the gas barrier effect is between 1.5 to 6% by weight and the amount of the block polymer (P) is from 0.1% to 0.5% by weight, with respect to the solid particles (p) in the fluid.
 2. The method according to claim 1, wherein the particles capable of providing a gas barrier effect (ii) are a latex.
 3. The method according to claim 2, wherein the latex is in the form of a suspension containing from 40% to 50% by weight of dry latex.
 4. The method according to claim 3, wherein the concentration of the block polymer (P) with respect to that of the dry latex is between 4% and 8% by weight.
 5. The method according to claim 1, wherein the latex is made of styrene/butadiene.
 6. The method according to claim 5, wherein the styrene/butadiene ratio by weight is between 30:70 and 70:30.
 7. The method according to claim 6, wherein the styrene/butadiene ratio by weight is between 40:60 and 60:40.
 8. The method according to claim 1, wherein the particles capable of providing a gas barrier effect (ii) are silica particles.
 9. The method according to claim 8, wherein the silica particles are in the amorphous form.
 10. The method according to claim 8, wherein the silica particles are dispersed or in the form of aggregates consisting of individual particles of 5 nm to 5 μm.
 11. The method according to claim 1, wherein the fluid (F) is an aqueous fluid and wherein the block (B) is a block at least predominantly composed of monomer units selected from the group consisting of the monomer units U1 to U5 defined below, and the mixtures of these monomer units: monomer units U1: monomer units comprising an acrylamide functional group, monomer units U2: monomer units comprising a sulfonic acid or sulfonate functional group, monomer units U3: neutral monomer units, esters of α,β-ethylenically unsaturated mono- or dicarboxylic acids with C₂-C₃₀ alkanediols or polyethylene glycols, and tetrahydrofurfuryl acrylate, vinylacetamide, vinylpyrrolidone, N-vinylpiperidone, N-vinylcaprolactam, N-vinyl-5-methyl-2-pyrrolidone, N-vinyl-5-ethyl-2-pyrrolidone, N-vinyl-6-methyl-2-piperidone, N-vinyl-6-ethyl-2-piperidone, N-vinyl-7-methyl-2-caprolactam or N-vinyl-7-ethyl-2-caprolactam, monomer units U4: monomer units carrying ammonium groups, including amides of α,β-ethylenically unsaturated mono- or dicarboxylic acids with diamines having at least one primary or secondary amine group; N,N-diallylamines or N,N-diallyl-N-alkylamines, monomer units U5: acrylate monomer units carrying a COOH or COO— group, wherein the block (B) optionally comprises hydrophobic monomers in a proportion of 0.05% to 10% by weight, with respect to the total weight of monomer units in the block (B).
 12. The method according to claim 11, wherein the block (B) optionally comprises hydrophobic monomers in a proportion of 0.05% to 10% by weight, with respect to the total weight of monomer units in the block (B).
 13. The method according to claim 1, wherein the fluid (F) is an oil cement grout which comprises the block polymer (P) as an additive.
 14. The method according to claim 13, wherein: the block (B) comprises monomer units U1 comprising an acrylamide functional group, and optionally units U2 comprising a sulfonic acid or sulfonate functional group; and the block (B) has a weight-average molecular weight of between 150 000 and 750 000 g/mol.
 15. The method according to claim 14, wherein the block (B) comprises dimethylacrylamide (DMA) units, and optionally units U2 comprising acrylamidomethylpropanesulfonic acid (AMPS), and/or has a weight-average molecular weight of between 200 000 and 700 000 g/mol.
 16. The method according to claim 1, wherein the second block (B) has a weight-average molecular weight of between 10 000 and 750 000 g/mol.
 17. The method according to claim 1, wherein the particles capable of providing a gas barrier effect are a latex and/or silica particles. 